At present, three-dimensional tissues are capable of being produced in vitro using various types of cells. For example, U.S. Pat. No. 5,443,950 issued to Naughton et al. describes three-dimensional cultures for bone marrow, skin, liver, vascular, and pancreatic tissues which are grown within synthetic matrices. In these tissues as well as others, investigators have been successful in proliferating cells and tissues in vitro such that the resulting three-dimensional tissues, termed "organoids" or "constructs", display many of the characteristics of their in vivo counterparts. These constructs have a variety of foreseeable applications, ranging from transplantation in vivo to functional and pharmacological testing in vitro.
In terms of muscle tissue, in vitro constructs of smooth muscle, cardiac muscle, and skeletal muscle have each been formulated. For example, U.S. Pat. No. 5,618,718 issued to Auger et al. describes the production of a contractile smooth muscle cell construct, and U.S. Pat. No. 4,605,623 issued to Malette et al. describes a method for cultivating the three-dimensional growth of cardiac myocytes. These smooth muscle and cardiac muscle constructs were each developed using mammalian muscle cells, specifically, human muscle cells.
In contrast, the majority of skeletal muscle organoids have been developed using avian muscle cells. In particular, a series of studies conducted by Vandenburgh and colleagues involved the production of organoids from avian muscle cells grown on an expandable, SILASTIC.RTM. membrane (Vandenburgh, In Vitro Cell. Dev. Biol. 24: 609-619, 1988; Vandenburgh et al., Am. J. Physiol. 256 (Cell Physiol. 25): C674-C682, 1989; Vandenburgh et al., In Vitro Cell. Dev. Biol. 25: 607-619, 1989; Vandenburgh et al., FASEB J. 5: 2860-2867, 1991). Since avian muscle is structurally and functionally distinct from mammalian muscle, organoids developed from avian muscle have no direct clinical application. A few skeletal muscle constructs have been developed using mammalian muscle grown within a synthetic matrix (Vandenburgh et al., Hum. Gene Ther. 7: 2195-2200, 1996; Shansky et al., In Vitro Cell Dev. Biol. 33: 659-661, 1997). However, the constructs in these studies originated from cells extracted from neonatal rats or immortal cell lines (C2C12) established from C3H mice which, due to their age or pathology, have limited clinical significance.
Previous methods of organoid production have additional drawbacks. First, in the majority of the studies by Vandenburgh and colleagues described above, as well as in U.S. Pat. Nos. 4,940,853 and 5,153,136, both issued to Vandenburgh, mechanical strain is applied to the skeletal muscle organoids for their proper development, such that complex mechanical fixturing and control electronics are required. Second, both the mammalian and avian skeletal muscle constructs have a limited in vitro life span of approximately four weeks, preventing their use for long-term functional or pharmacological studies.
Perhaps the most serious drawback of previous studies involving the growth of three-dimensional tissues is that the type of anchor systems to which the tissues attach restricts the ability to functionally evaluate the tissues. For instance, when a synthetic membrane or matrix is utilized, the contractile function of the organoids may be difficult to determine separate from the matrix material due to the mechanical preloads of the matrix material. When synthetic anchors such as stainless steel pins or mesh are employed, the tissue merely grows around the anchors instead of into them, such that there is a large discontinuity in mechanical impedance. This discontinuity creates a stress concentration, which could lead to cell damage when the tissue contracts.